The enhanced barrier performance is obtained by application of inorganic coatings to the external surface of the container. The coatings exhibit enhanced adhesion relative to prior art coatings.
Plastic containers currently comprise a large and growing segment of the food and beverage industry. Plastic containers offer a number of advantages over traditional metal and glass containers. They are lightweight, inexpensive, nonbreakable, transparent and easily manufactured and handled. However, plastic containers have at least one significant drawback that has limited their universal acceptance, especially in the more demanding food applications. That drawback is that all plastic containers are more or less permeable to water, oxygen, carbon dioxide, and other gases and vapors. In a number of applications, the permeation rates of affordable plastics are great enough to significantly limit the shelf-life of the contained food or beverage, or prevent the use of plastic containers altogether. Shelf-life is the time needed for a loss of seventeen percent of the initial carbonation of a beverage.
It has been recognized for some time that a container structure that combines the best features of plastic containers and more traditional containers could be obtained by applying a glass-like or metal-like layer to a plastic container, and metallized plastic containers. For example, metallized potato chip bags have been commercially available for some time. However, in a number of applications, the clarity of the package is of significant importance, and for those applications metallized coatings are not acceptable. Obtaining durable glass-like coatings on plastic containers without changing the appearance of the container has proven to be much more difficult.
A number of processes have been developed for the purpose of applying glass-like coatings onto plastic films, where the films are then subsequently formed into flexible plastic containers. However, relatively few processes have been developed that allow the application of a glass-like coating onto a preformed, relatively rigid plastic container such as the PET bottles commonly used in the U.S. for carbonated beverages, and heretofore no process has been developed that allows the application of a glass-like coating onto the external surface of a plastic container that is sufficiently durable to withstand the effect of pressurization of the container, retain an enhanced barrier to gases and vapors subsequent to said pressurization, and not affect the recyclability of the containers. Pressurized beverage containers currently comprise a very large market world-wide, and currently affordable plastics have sufficiently high permeation rates to limit the use of plastic containers in a number of the markets served.
Such pressurized containers include plastic bottles for both carbonated and non-carbonated beverages. Plastic bottles have been constructed from various polymers, predominant among them being polyethylene terephthalate (PET), particularly for carbonated beverages, but all of these polymers have exhibited various degrees of permeability to gases and vapors which have limited the shelf life of the beverages placed within them. For example, carbonated beverage bottles have a shelf-life which is limited by loss of CO2. While this limitation becomes increasingly important as the size of the bottle is reduced, because of the increasing surface to volume ratio, small containers are needed for many market applications, and this severely limits the use of plastic bottles in such cases. Generally, based upon this surface to volume ratio, as a bottle becomes smaller, carbonation retention in the beverage becomes more difficult.
For non-carbonated beverages, similar limitations apply, again with increasing importance as the bottle size is reduced, on account of oxygen and/or water-vapor diffusion. It should be appreciated that diffusion means both ingress and egress (diffusion and infusion) to and from the bottle or container. The degree of impermeability (described herein as xe2x80x9cgas barrierxe2x80x9d) to CO2 diffusion and to the diffusion of oxygen, water vapor and other gases, grows in importance in conditions of high ambient temperature. An outer coating with high gas barrier can improve the quality of beverages packed in plastic bottles and increase the shelf life of such bottles, making small bottles feasible, and this in turn presents many advantages in reduced distribution costs and a more flexible marketing mix.
Some polymers, for example PET, are also susceptible to stress cracking when they come in contact with bottle-conveyor lubricants used in bottle filling plants, or detergents, solvents and other materials. Such cracking is often described as xe2x80x9cenvironmental stress crackingxe2x80x9d and can limit the life of the bottle by causing leaks and prevent damage to adjacent property. An impermeable outer surface for plastic bottles, and prevent damage to adjacent property which resists stress-cracking-inducing chemicals, will extend the shelf life of plastic bottles in some markets.
Another limitation to shelf life and beverage quality is often UV radiation which can affect the taste, color and other beverage properties. This is particularly important in conditions of prolonged sunshine. An outer coating with UV absorbing properties can improve the quality of such beverages and make plastic bottles much more useable under such conditions.
Additional functionality can be incorporated into the inorganic coating by incorporating visible light absorbing species, rendering the plastic container cosmetically more appealing.
An additional benefit of the present invention is ease of recycling. Prior art barrier enhancing coatings generally are organic in nature and are much thicker than the coating of the present invention. Consequently, when post-consumer scrap containing containers coated with prior art organic coatings are recycled, significant deterioration in the appearance and properties of the plastic occur. In contrast, because of the inert nature and thinness of the coatings of the present invention, the coated containers can be processed in any conventional recycling system without modification of the process.
Moreover, haziness in recycled articles can occur when large sized particles are used in a coating. However, such a haze is avoided in the present invention because relatively small particles are used as will be described. Moreover, the coating is acceptable for food contact and therefore will not adversely affect the recycling effort when ground or depolymorized in the recycling process.
Along the lines of recycling, the present invention provides for a method of recycling coated plastic which has results heretofore unattainable. In particular, separation of coated and uncoated plastics is unnecessary whereby modifications to existing recycling systems are unnecessary or whereby extra process steps (separating coated bottles from uncoated bottles) can be avoided. Moreover, it is possible to produce a transparent plastic from coated plastic while avoiding the above-noted problem of haziness in the final recycled product. While the present invention can be used in recycling many types of plastic, it is contemplated that this invention can be used with plastic articles, such as containers or bottles and more particularly, with plastic beverage bottles. Bottle-to-bottle recycling remains unaffected with the present invention.
Recycling in the instant invention can be carried out in both a chemical process and a physical process. The plastic subjected to recycling processes can be molded or extruded. Even if a coated plastic is initially introduced in the recycling process, the coating of the present invention will not interfere with the downstream injection molding or blow molding.
Finally, the cost of applying a coating to the outside of a bottle, which has a gas barrier which significantly increases the shelf-life of beverage container in that bottle, and/or which significantly reduces product spoilage of beverage container in that bottle, and/or which significantly reduces product spoilage due to UV radiation, and/or virtually eliminates environmental stress cracking, and/or provides a specific color, must not add significant cost to the basic package. This is a criterion, which eliminates many processes for high gas barrier coatings, because plastic bottles are themselves a very low cost, mass produced article. Affordability implies in practice that the cost of the coating must add minimal or no increase to the cost of the whole package and in fact, the cost can be less.
A coating on the outside of plastic bottles must be capable of flexing. When bottles are used for pressurized containers, the coating preferably should be able to biaxially stretch whenever the plastic substrate stretches. In addition it is preferable that the coating be continuous over the majority of the container surface. However, in the present invention, unlike prior art devices, the continuous coating is not essential because of the high level of adhesion of the inorganic coating to the surface of the plastic container. In other words, even though the coating of the present invention may be non-continuous because of scratches or fractures therein, for example, the coating will continue to effectively adhere to the substrate such as an underlying plastic bottle. The present invention can therefore provide an effective gas barrier even if the surface is highly fractured. A high gas barrier of 1.25xc3x97 greater than a similar uncoated container can be obtained with the present invention and this barrier can even be 1.5xc3x97 or preferably 2xc3x97 greater.
Adhesion is particularly important in the case of carbonated beverages, since the CO2 within the bottle exerts some or all of its in-bottle pressure on the coating. This pressure can rise to above 6 bar, exerting considerable forces on the coating/plastic interface. The coating must also resist scuffing, normal handling, weathering (rain, sun climate, etc.), and the coating must maintain its gas barrier throughout the bottle""s useful life.
There are several plasma-enhanced processes which apply an external, inorganic coating to a range of articles, which in some cases includes bottles. Many of the processes are targeted to provide coating properties which are quite different, and far less onerous than high gas barrier bottle coatings. Such processes target, for example, abrasion resistance, where the coating continuity is not a major factor, since the coating can protect the microscopic interstices. Other processes target cosmetic or light-reflection properties and some processes have a pure handling protection role. Often the substrate does not flex nor stretch and the article itself is higher priced than plastic bottles so that cost is not a benefit of the design. In some cases, the substrate allows far higher coating temperatures than those allowed by PET, the most common plastic-bottle material. Such processes do not, in general, provide the coating continuity, adhesion, flexibility needed for high gas barrier coatings, nor do they provide a solution to the other problems relating to high gas barrier coatings, described above.
Prior art also exists for gas barrier processes for bottles, but the lack of commercially available, coated bottles for pressurized application is due to the fact that these processes lack the desirable attributes described above and fail to provide a coating with adequate adhesion, continuity and/or flexibility under high in-bottle pressure or a coating which avoids recycling problems, or the low cost necessary to make the coating affordable.
U.S. Pat. No. 5,565,248 to Plester and Ehrich describes a method for coating containers internally. However, external coatings require far greater adhesion than internal coatings, because in-bottle pressure acts against external coatings, and internal coatings are not subject to the same handling and/or abrasion in use. For these, and other reasons, coating bottles externally differs from coating them internally and the present invention is therefore substantially different.
Accordingly, it is an object of the present invention to provide an outer coating or layer for a container such as a heat sensitive plastic bottle, and particularly for the non-refillable bottles used for carbonated beverages.
It is a further object of the present invention to provide a coating and a system and method for coating which can provide an external glass-like coating that is flexible, durable and possess sufficient adhesion to withstand the effects of pressurization without significant loss of enhanced barrier properties.
An additional object of the present invention is to provide an externally coated container which will avoid environmental stress cracking such as when the container comes into contact with conveyor lubricants during filling, detergent, cleaners or solvents or similar substances during its life cycle. Such lubricants can include 409(trademark), Mean Green(trademark) or other commercially available cleansers or lubricants, etc.
Yet another object of the present invention is to provide a lighter container and a system and method for making the container whereby an amount of plastic utilized in making the container as compared to a conventional container can be reduced without adversely effecting or while improving the gas barrier effectiveness of the container.
It is another object of the present invention to provide a coating that comprises an inorganic oxide layer on the external surface of a plastic container, the inorganic oxide layer being further distinguished by being comprised of greater than or equal to 50 and up to but less than 100% SiOx (x=1.7 to 2.0).
Another object is to provide a coating which possesses sufficient adhesion to the external surface of the plastic container so that the barrier enhancement provided by the inorganic oxide layer is not substantially reduced upon pressurization of the container to a pressure between 1 and 100 psig.
A further object of the present invention is to provide a method for applying an inorganic layer as described above, the method resulting in a robust inorganic oxide layer that provides an effective level of barrier enhancement to the plastic container and does not result in significant physical distortion of the container.
It is a further object of the present invention to provide a system and method for manufacturing a container whereby the aesthetic appeal of the container will be enhanced by applying a colored inorganic layer that further contains visible-light absorbing species.
Yet another object of the present invention is to provide a coating for a container with UV absorbing capabilities.
Still another object of the present invention is to provide a container with a colored or clear coating which can easily be recycled without significant or abnormal complications to existing recycling systems.
Another object of the present invention is to provide a system and method for inexpensively manufacturing an externally coated container.
Yet another object of the present invention is to provide a method which the thickness and composition of the applied coating on a container can be rapidly and easily determined and whereby process control and insurance of enhanced barrier performance can be obtained.
A further object of the present invention is to provide a method to determine the condition of the surface of a plastic container at least with regards to its suitability for applying glass-like coatings.
Another object of the present invention is to provide a high gas barrier which considerably increases the shelf life of the containers such as plastic bottles and to provide the containers with good transparency so as not to affect the appearance of a clear plastic bottle.
Yet another object of the present invention is to provide a container with enhanced barrier performance both when the container walls flex or stretch under pressure and when the walls are indented.
Still another object of the present invention is to provide a container with adequate durability and adhesion during working life, when the outer surface of the container is subjected to environmental conditions such as severe weather, rubbing, scuffing, or abrasions (for example, during transportation) or when the outer coating is subjected to internal pressure and with the ability to maintain a gas barrier while remaining stressed by in-bottle pressure throughout the container""s useful life.
Also, another object of the present invention includes the ability to enable coating to heat sensitive plastic containers with coating materials, which can only be vaporized at very high temperatures without an acceptable increase in the plastic""s temperature and which must remain in many cases below 60xc2x0 C.
The foregoing and other objects of this invention are fulfilled by a method, apparatus and process control procedure for plasma-assisted deposition of a very thin, typically 10-100 nm, inorganic outer surface coating on a container such as a plastic bottle using inorganic substances.
Although not bound to any particular theory, the formation of a highly adherent, dense, continuous coating may be achieved in the present invention by producing a high energy plasma which further provides good coating adhesion by enabling penetration of the coating beneath the surface of the plastic. This penetration is additionally assisted by a biasing energy using RF or HF. Formation of a dense coating is further enhanced by the ability to coat under conditions of high vacuum (generally in range 10xe2x88x923 to 10xe2x88x925 mbar) which avoids unwanted gas molecules being incorporated into the coating and is still further enhanced by the ability to cause the reaction components of the coating to react stoichiometrically.
Furthermore, the method described herein enables surface pretreatment for the activation of the plastic surface by forming free radicals which can react with and attach to the coating and enhance adhesion before coating begins. The method further includes surface cleaning, where necessary. The method identifies the need to apply surface activation during pretreatment, and to control this, since activation can be counter productive by damaging the surface. The method also describes means of inspecting the bottle surface for coating suitability and identifies factors which lead to the preference to coat containers (particularly PET bottles) quickly, or immediately, after molding. The method describes means of degassing the plastic, so as to avoid vapor emission from the plastic surface which interferes with the coating, and reduces its adhesion or density.
The form of deposition provides for an approach angle of plasma particles to the surface which does not exceed 70xc2x0 to the vertical, so as to enhance coating adhesion. It also provides for mixtures of substances to be deposited, and particularly for the trace of addition (up to 50%) of metal ions into silica, which increases silica""s gas barrier for pressurized packages. The form of deposition also enables heat sensitive containers to be coated without significant temperature rise, and at all times maintaining a bottle temperature well below 60xc2x0 C.
While this angle of 70xc2x0 to the vertical was noted above, it is important to recognize that there may be certain situations where this limitation would not apply. For example, if bottles or containers surrounded a vapor source, then no limitation on the angle would be desired. In other words, plasma particles could be moving 360xc2x0 from their source if the bottles or containers to be coated surrounded the source. Alternatively, if two parallel conveyors were provided for moving the bottles or containers past a given source with the source being between the conveyor lines, then plasma particles could also move 360xc2x0 from this source in order to reach the bottles or containers on both conveyor lines. Other situations are envisioned whereby the angle would not be limited to 70xc2x0.
The method enables mixtures and layers of substances to be applied which can be chosen for their color, or UV-absorbing properties, or additional gas barrier properties. Further, the method enables coatings, such as silica, which are fully transparent and clear, and would therefore not affect the appearance of an otherwise clear bottle. The coating materials are inert and remain solid when the plastic bottle is melted for recycling.
The method for coating the outer surface of a container according to the present invention comprises the steps of (1) conveying the containers to a loading/unloading station where they are placed into a holding crate (xe2x80x9cholderxe2x80x9d); (b) conveying the holder/containers into a xe2x80x9ctoolxe2x80x9d station which applies in-bottle antennas and seals the containers with caps, which enable the inside of the container to stay under pressure during coating and provide for the containers to be gripped, rotated and released at the appropriate parts of the coating cycle; (c) locating holder/containers in an evacuation cell which brings the holder/containers to the pressure of a vacuum cell; (d) conveying holder/containers into the vacuum cell to a container loading/unloading table and raising holder/bottles to permit the containers to be gripped and located in a conveyor chain; (3) conveying containers while continuously rotating them in a vertical position within the vacuum cell through stages, which enable degassing of plastic, then pretreatment to clean/activate the container surface, then container base coating; (f) conveying containers while continuously rotating them in a horizontal position within the vacuum cell to enable the container sidewall to be coated; (g) returning the coated containers to the container loading/unloading table where they are replaced in the holder; (h) returning holder/containers to the evacuation cell where they are brought back to normal atmospheric pressure; (i) conveying the holder/containers to the xe2x80x9ctoolxe2x80x9d station where the antennas and caps are removed; (j) conveying holder/containers to the container loading/unloading station where the coated containers are removed from holder and replaced by uncoated containers so that the cycle can be repeated.
Within the vacuum cells, a plasma at very low pressures is created using a conventional electron gun. While the gun itself is known, the method of using this gun in the present invention is new as will be described below. Trace metals can be added to a silica coating by sacrificial erosion of the electron gun. The solid deposit will stoichiometrically and on-surface react with the reactive gas to ensure that surplus gas molecules cannot be build into the coating, thus avoiding porosity. The coating is at high vacuum to reduce interference of ambient gas molecules with the coating process and to avoid gas molecules being built into the coating, causing porosity. A very thin coating, generally below 100 nm is deposited on the container. Optionally, RF and HF energy is used to bias the coating particles and an in-cell antenna and RF or HF or DC energy is optionally used to create a pretreatment plasma within the vacuum cell. The plastic container is degassed to avoid vapor emerging from the plastic which can interfere with the coating process. Multiple plasma-making systems can be used whenever more than one solid material is needed for the coating process.
An apparatus for performing the above-mentioned method steps comprises: (a) a holder and means of loading containers into it; (b) a cap which seals the container opening and which incorporates a screw driver-type slot and quick-snap-in-release connection for gripping, releasing and turning the containers; (c) an antenna which can be erected inside the container, maintain a minimal gap with the walls of the container without touching the walls, and can be orientated to face in the desired direction either magnetically or by gravity; (d) means of inserting antenna into the containers and applying the cap; (e) means of sealing holder/containers in a cell which can either be pumped down to coating pressure so as to enable the holder/containers to enter the vacuum cell, or can be repressurized to enable holder/containers to exit the vacuum call; (f) means of conveying holder/containers into the vacuum cell and means for gripping the containers in a conveyor system which runs within the vacuum cell; (g) a vacuum cell with an internal conveying system which conveys the containers, while continuously rotating them, first in vertical position, then in horizontal position; (h) a plasma-making system having a conventional electron-gun and coating-materials that are to be deposited; (i) a biasing system using RF or HF energy and applying it to the in-container antenna, and use of this system both for pretreatment and for densifying the coating by biasing the coating particles; (j) an optional in-vacuum cell mounted antenna which can create a pretreatment plasma by using RF or HF or DC energy; and (k) means of introducing a gas or mixture of gases into the vacuum cell.
These and other objects of the present invention are fulfilled by a system for coating an external surface of a container wherein the container on pressurization possess a gas barrier at least 1.25xc3x97 greater than a similar uncoated container, the system comprising:
a vacuum cell, pressure within the vacuum cell being reduced as compared to ambient pressure;
means for supplying containers to and withdrawing containers from the vacuum cell; and
at least one source for supplying a coating vapor to the external surface of the containers in the vacuum cell, the coating vapor from the at least one source depositing a relatively thin coating on the external surface of the containers,
whereby bonding between at least a portion of the relatively thin coating deposited on the container and the external surface of the container occurs.
Moreover, these and other objects of the present invention are fulfilled by a system for coating an external surface of a container wherein the container on pressurization has enhanced environmental stress crack resistance, the system comprising:
a vacuum cell, pressure within the vacuum cell being reduced as compared to ambient pressure;
means for supplying containers to and withdrawing containers from the vacuum cell; and
at least one source for supplying a coating vapor to the external surface of the containers in the vacuum cell, the coating vapor from the at least one source depositing a relatively thin coating on the external surface of the containers,
whereby bonding between at least a portion of the relatively thin coating deposited on the container and the external surface of the container occurs.
In addition, these and other objects of the present invention are fulfilled by a plastic container having an inorganic oxide coating on an external surface thereof, the coated plastic container possessing a gas barrier of at least 1.25xc3x97 after pressurization as compared to a similarly pressurized uncoated container.
These and other objects of the present invention are also fulfilled by a method for coating a plastic container comprising the steps of:
introducing said container into a vacuum cell;
applying an inorganic composition to an external surface of the container in the presence of at least one reactive gas and at sub-atmospheric pressure; and
removing the container from the vacuum cell.
In addition, these and other objects of the present invention are fulfilled by a method for producing recycled content plastic comprising the steps of:
providing a batch plastic, at least a portion of the plastic having a coating thereon;
grinding the plastic to produce flakes; and
melt extruding the flakes.
A method for producing recycled content plastic fulfills these and other objects of the present invention with the steps of:
providing at least some coated plastic;
depolymerizing said at least some coated plastic; and
re-polymerizing said depolymerized plastic.
Moreover, these and other objects of the present invention are fulfilled by a plastic container having an exterior inorganic coating that has an equivalent gas barrier and reduced weight compared to a plastic container of similar surface area and volume and without said exterior inorganic coating.
In addition, these and other objects of the present invention are fulfilled by a method of extending shelf life of a plastic container comprising the steps of:
providing a plastic container, the plastic container having an inorganic coating on an external surface thereof;
filling the plastic container with a pressurized beverage;
sealing the plastic container after the step of filling;
preventing escape of pressurized gas from the plastic container, the coating being utilized in prevention of escape of the pressurized gas;
holding the pressurized gas within the container for a longer time period than a similarly sized and shaped container without the inorganic coating, the inorganic coating being utilized in the holding of the pressurized gas; and
permitting at least one of flexing and stretching of the inorganic coating without substantially affecting the step of holding.
Still yet these and other objects of the present invention are fulfilled by a device for coating containers, the device comprising:
an electron gun;
a receptacle for holding material, the electron gun vaporizing at least a portion of the material held in the receptacle; and
means for conveying containers through an area having vapor produced by the electron gun vaporizing the material, the vapor being deposited on the containers to thereby coat an exterior of the containers.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.